610 



MOLLUSCA 



PHYLUM VI 



of each segment towards the interior. These internal calcareous septa are united only in 

 their 'peripheral I'tarts, not meeting at the central axis so far as known, and also liable 

 to be more or less interrupted in the transverse plane of each funnel. The interior is 

 consequently an actiniform endosipihuncle toith rays extending outwardly between the 

 laminae of the deposits. 



Family 4. Jovellanidae Hyatt. 



Orthoceracones and cyrtoceracones with slightly compressed oval, or depressed and 

 more or less sub-trigonal sections. Shells smooth or partially annulated. Siphuncle large, 

 with well-developed actiniform. lamellae, and distinct endosiphuncles. Aperture open and 

 living chamber uncontracted in the gerontic stage. 



Includes Jovellania Bayle ; and Tripleuroceras Hyatt. Silurian and Devonian. 

 Mixosiphonoceras Hyatt. Type M. {Gyrtoceras) desolatum (Barr.). Silurian and 

 Devonian. Projovellania Hyatt. Type P. (Gyrtoceras) athleta (Barr.). Silurian. 



Family 5. Rizoceratidae Hyatt. 



Orthoceracones and cyrtoceracones expanding regularly by growth throughout life, the 

 living chamber very slightly or not sensibly contracted in the gerontic stage. Aperture con- 

 stantly open, and with slight dorsal as well as somewhat deeper and 

 broader hyponomic sinuses. Siphuncle generally small and empty, 

 but actiniform lamellae and an endosiphuncle sometimes occur. 

 Shells as a rule smooth or with transverse bands only, hut longitu- 

 dinal striae are often present in earlier stages. 



Rizoceras Hyatt (Fig. 1131). Orthoceracones and exo- or 

 endogastric cyrtoceracones having circular or elliptical sections. 

 Living chamber extraordinarily large and long as compared with 

 camerated part. Silurian to Carboniferous. 



Cyrtorizoceras Hyatt. Sections more com- 

 pressed than in Rizoceras, living chamber 

 shorter and apt to be more or less laterally 

 compressed in gerontic stage, but the dorso- 

 ventral diameters only very slightly so or not 

 at all. Sutures more sinuous, and vi^ith decided 

 ventral and dorsal saddles. Type C. (Gyrtoceras) 

 minneapolis (Clarke). Ordovician and Silu- 

 rian. 



Fig. 1131. 



Rizoceras rohustum 

 (Barr.). Silurian (E) ; 

 Butovvitz, Bohemia. 

 Aperture open. i/.j. 



Family 6. Ooceratidae Hyatt. 



Orthoceracones and gyroceracones with closely set septa and yig 1132. 



large nummuloidal siphuncle in later stages of the ontogeny, hut ooceras (Ci/rtuceras) 



tubular in the younq. Actiniform. deposits oftener nresent than in ''«?/'ei (Barr.). Silurian 



J, Ti- s-1 i^ i 7 T. 7 -77 . (E); Lochkow, Bohemia 



the mzoceratiaae, but not generat. Jiunnels very variable, sometimes (after Barrande). 

 minutely plicated or hook-like in section, confined to dorsal side of 

 tube, or sometimes absent altogether. Living chamber short and like that of Cyrto- 

 rizoceras ; aperture not infrequently suh-trigonal in outline, but always open. 



Ooceras Hyatt (Oonoceras Hj^att) (Fig. 1132). Cyrtoceracones more elongated 

 and usually more compressed than in Gyrtorizoccras, but otherwise similar except in 

 structure of the siphuncle. Septa rise rapidly on ventral side, and may bend sharply 

 orad, forming a funnel ridge or shoulder on tliat side, but disappearing on the 



